A method for increasing the transmission capacity of optical fiber communication networks without establishing more optical transmission lines is wavelength-division multiplexing. In these systems generally known as wavelength-division multiple transmission systems, a plurality of signal light beams having wavelengths different from one another are employed to optically transmit the resultant optical signals.
In these wavelength-division multiplexed networks constituent light beams can incur a dispersion between their intensity or phase. An intensity dispersion can be caused by a gain dispersion of an optical amplifier employed in a middle and long distance optical fiber communications system for amplifying a signal light beam which has been attenuated through a transmission line fiber. As the optical amplifier, there is widely used an optical fiber amplifier for amplifying directly a signal light beam with a rare-earth doped optical fiber as an amplification medium. Also, as the amplification medium of the optical fiber amplifier, the rare-earth doped optical fiber is employed. In this connection, there is generally known an EDFA (an Erbium Doped Fiber Amplifier) employing erbium elements as dopant.
However, if the constituent signal light beams of the wavelength division multiplexed signal light beam are intended to be optically amplified by the optical fiber amplifier, the dispersion between the gains for the constituent signal light beams due to the wavelength characteristics of the gain of the erbium doped optical fiber, i.e., the degradation of the flatness of the gains becomes a problem. For the purpose of improving the gain flatness, an optical filter is employed.
The introduction of optical amplification in wavelength-division multiplexed networks enables longer transmission distances. However, with increased transmission distances and higher bit rates per signal channel, fiber dispersion becomes a problem. Fiber dispersion causes the pulses to broaden. If the fiber dispersion is large enough, pulses in constituent signal channels may overlap in time and cause loss of transmitted information. Thus, filters are needed for performing phase dispersion compensation.
All the above mentioned filter functions can be performed by filters consisting of a concatenation of tunable couplers and tunable delay lines forming a so-called finite impulse response (FIR) filter.
Optical transmission systems, such as optical couplers, are formed from a plurality of waveguides which each include a core and a cladding. In the optical coupler, two or more waveguides are arranged adjacent to and closely separated from one another. An optical coupler splits lightwaves coherently in a manner similar to a beam splitter in bulk optics. The evanescent tail of the lightwave in one waveguide extends to the neighboring waveguide and induces electric polarization. The polarization generates a lightwave in the second waveguide which also couples back to the first waveguide in a well known manner. For a waveguide coupler, the fraction of the light coupled from the first waveguide to the second waveguide is known as the coupling ratio. The coupling ratio is dependent on a number of factors, including the refractive indices of the core and cladding of the waveguides and the separation between the waveguides. However, one limitation in the fabrication of optical couplers having coupling ratios that are precisely specified is that process control of the refractive indices of the cladding and core is often not sufficient to result in high yields for designs having such precise requirements.
Mach-Zehnder interferometers are known and are expected to find use, inter alia, in dense wavelength-division multiplexed optical communication systems. Such systems will typically require the ability to passively multiplex and de-multiplex channels at the link ends and, at least in some architectures, to add and/or drop channels at selected points on the link. These abilities can be provided by the above mentioned Mach-Zehnder devices, especially by such devices that comprise refractive index-gratings in both arms of an equal arm Mach-Zehnder-type wavelength interferometer.
Such devices, in order to provide acceptable performance, have to meet exact requirements on, e.g., equality of arm lengths and equality of grating strengths. R. Kashyap et al., IEEE Photonics Technology Letters, Vol. 5(2), p. 191 (February 1993), disclose a Mach-Zehnder-type interferometer fabricated in Ge-doped planar silica. Planar waveguide Mach-Zehnder-type interferometers can be relatively easily manufactured with essentially equal arm length, due to the close dimensional control obtainable with standard photolithography and etching techniques. Nevertheless, Kashyap et al. found imbalance in the arms that had to be compensated by trimming. This compensation was achieved by laser trimming of one photosensitive arm of the interferometer.
U.S. Pat. No. 5,768,452 discloses a method of trimming the coupling ratio of an optical coupler to a prescribed value. The optical coupler is formed from a plurality of waveguides. In accordance with this method, an irradiation energy is selected that is absorbed by portions of the waveguides located in a coupling region. A dosage of radiation is applied to the waveguide portion at least sufficient to adjust the optical coupling ratio to the prescribed value. The radiation, which may be absorbed by the cladding and/or core of the waveguides, causes a change in the refractive index difference between the core and cladding of the waveguides. This change in the refractive index difference will result in a change in the optical coupling ratio of the device. The respective optical coupler is stabilized so that the induced change in refractive index, and hence the value of the coupling coefficient, does not undergo substantial decay over time. Such stabilization is done by thermally annealing the coupler after exposing it to radiation.
Because of space limitations in planar waveguide technology, smaller bending radii compared to traditional fiber technology are required. Smaller bending radii, however, require a stronger guiding of the optical modes than in a straight waveguide or fiber. This is achieved by increasing the refractive index contrast between core and cladding as compared to the co-planar waveguide technology, which, however, leads to increased coupling losses. A good compromise between the minimum bending radius and coupling losses to the standard fiber and the necessary difference in the material composition between core and cladding resulting therefrom can be obtained, e.g., with an effective refractive-index contrast around 0.02. If the cladding is made of silica, i.e., SiO2, which has a refractive index of 1.45, a material having a refractive index near 1.51 is desired for the core. However, limitations in the maximum achievable refractive-index-change by doping with P, Ge or other dopants and hence in the minimum attainable bending radius in the waveguide exist.
Waveguides with a much higher index contrast can be fabricated with silicon-oxinitride (SiON) core layers. An example for the use of SiON as a material for fabricating waveguides is given in U.S. Pat. No. 5,416,861.
An example for a method for increasing the index of refraction of a glassy material is disclosed in U.S. Pat. No. 5,500,031, wherein the material is treated with hydrogen under the application of heat. That method is not used to increase the index contrast of the material in general, i.e., all over the wafer, but would be used in local areas to compensate for fabrication inaccuracies or make custom changes to a more general design. The hydrogen incorporation, however, causes an increase of the propagation loss.
A typical fabrication technique is to deposit silicon oxinitride by means of a PECVD process using silane (SiH4), nitrous-oxide (N2O), and ammonia (NH3) as gaseous precursors. Thus, hydrogen is embedded in SiON, which is then driven out to a great extent and replaced by nitrogen by means of two subsequent annealing processes (core and cladding).
Dianov et al., “Grating Formation in a Germanium Free Silicon Oxynitride Fiber”, Electronics Letters, Vol. 33, No. 3, p. 236 ff., January 1997, disclose writing Bragg gratings in a germanium-free nitrogen-doped-silica-core fiber. The single mode fiber used was manufactured by hydrogen-free reduced-pressure surface plasma-chemical vapor deposition (SPCVD).
An optical signal processor being represented by an optical circuit of a lattice configuration is disclosed in U.S. Pat. No. 5,572,611. Its basic circuit structure comprises 3 dB directional couplers, two optical waveguides with equal optical path lengths, and two optical waveguides with different optical path lengths (an optical path difference of about 1 to 50 mm). Phase controllers for performing phase shift are provided on the optical waveguides. The portions with equal optical path lengths function as variable directional couplers, and variable directional couplers having arbitrary coupling rates can be constructed by changing the phase controllers on the optical waveguides with equal optical path lengths. As for an adaptive filter, there is adopted a construction in which a photodetector for withdrawing part of output is provided at the output port, and a feedback electric wiring for feedback control is laid.
The components to change the intensity, phase or path of the constituent signal light beams may consist of a concatenation of tunable couplers and tunable delay lines forming a so-called finite impulse response (FIR) filter. An example of such a device 6 is given in FIG. 1, where symmetric Mach-Zehnder interferometers 2 are depicted, together with asymmetric Mach-Zehnder interferometers 4.
With the help of chromium heaters on one arm of each interferometer an additional phase shift between the two arms can be induced and thus the coupling ratio and the frequency response of the tunable coupler and the tunable delay line, respectively, can be changed. This enables the construction of a desired total frequency response of the FIR filter. Usually the FIR filter would be designed to have a certain frequency response for the off-state of all heaters. However, due to small phase errors induced by inhomogeneities on the chip often this response cannot be achieved.
Thus, there is a need for trimming such a FIR filter of planar waveguides. In T. Erdogan, V. Mizrahi, P. J. Lemaire and D. Moore, “Decay of ultraviolet-induced fiber Bragg gratings”, J. Appl. Phys. 76(1), 1994, it is reported that the use of a chromium heater close to a standard Ge-doped glass waveguide that is treated with UV-light would destroy a trimming as mentioned above over time, because the stability of the refractive index change is significantly lower at elevated temperatures. Hence, the stability of this effect with temperature would decrease very quickly.
Hence there is still a need to provide a trimming method that allows to operate the trimmed device at elevated temperatures without incurring stability problems.